1. Technical Field
The present invention relates to a thermal head and to a thermal printer that uses the thermal head.
2. Related Art
Thermal printers that print by conveying thermal paper or other print medium enabling thermal printing over a thermal head having heating elements disposed thereto are known from the literature. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2006-88584.
FIG. 8 is a section view of the print unit in the thermal printer 201 described in JP-A-2006-88584. The thermal head 220 disposed in this thermal printer 201 is pushed to the platen roller 210 side by a coil spring 206, and the print medium P is thereby held between the platen roller 210 and the thermal head 220. This type of thermal printer 201 prints by causing the print medium P to change color by applying heat thereto by means of the thermal head 220.
When this thermal printer 201 according to the related art prints for an extended period of time to a low quality, coarse print medium P with high surface roughness, parts of the common electrodes 225 may wear and fail as a result of the print medium P repeatedly wearing a particular part of the common electrode 225 of the thermal head 220 (see FIG. 9), eventually resulting in an inability to print.
To further describe this problem, FIG. 9 shows a top view of the main parts of a common thermal head 220. The contact surface 211a pressed by the platen roller 210 against the thermal head 220 is indicated by a double-dot dash line in FIG. 9.
As shown in FIG. 9, a heat unit 221 having a plurality of heat elements 221a arrayed in a line is formed on the substrate 223 of the thermal head 220. A plurality of drive electrodes 224 that supply drive current to the heat elements 221a are formed on the substrate 223 on one side of the linear heat unit 221, and are connected to a drive chip not shown.
A common electrode 225 that is conductive with each of the heat elements 221a is also formed on the substrate 223 on the other side of the heat unit 221. The common electrode 225 communicates with the drive electrode 224 side through a electrode connection unit 226 that is formed at the end of the heat unit 221 array, and is connected to an external connector not shown.
The rotational axis Ax of the platen roller 210 is disposed opposite the thermal head 220 aligned with the alignment axis of the plural heat elements 221a so that the print medium P can be efficiently pressed against the heat unit 221, and is affixed to the frame of the thermal printer 201 not shown. The print medium P is held between the platen roller 210 and the thermal head 220 as a result of the thermal head 220 being pushed to the platen roller 210 side by the coil spring 206.
The width of the platen roller 210 is greater than the width (the left-right direction in FIG. 8) of at least the heat unit 221 so that the print medium P can be reliably pressed against the heat unit 221. As a result, the platen roller 210 is pressed through the intervening print medium P to the heat unit 221 and the electrode connection unit 226 that is disposed on the axial end 221c side of the heat unit 221. While the thermal head 220 and platen roller 210 meet at the contact surface 211a, pressure is particularly great on the area 211b of the contact surface 211a that is closest to the rotational axis Ax because the platen roller 210 is a cylinder centered on the rotational axis Ax.
The common electrode 225 including the electrode connection unit 226 is thicker than the drive electrodes 224 and the heat elements 221a in order to carry the combined current flowing from the plural heat elements 221a. A protective coating is also formed over the electrode connection unit 226 and the heat elements 221a. However, as the protective coating on the electrode connection unit 226 is worn by the print medium P, the electrode connection unit 226, which is softer than the coating, becomes worn in spots. More particularly, as shown in FIG. 10, the part 226a of the electrode connection unit 226 that is opposite the pressure area 211b of the platen roller 210 becomes worn as shown in FIG. 10.
As the electrode connection unit 226 continues to wear and the common electrode 225 finally fails in this part 226a of the electrode connection unit 226, conductivity is lost between the external connector and the common electrode 225, and the heat unit 221 cannot be driven. The thermal printer 201 thus becomes unable to print when a low quality, coarse print medium P is used for a long time.